A hidden observation post, disguised by a hologram or cloak, from which Federation researchers study a pre-warp society without the inhabitants knowing they are being watched.
The duck blind is the Federation's standard tool for studying cultures protected by the Prime Directive. In the Next Generation episode Who Watches the Watchers, an anthropological team on Mintaka III works from a post hidden behind a holographic rock face. A reactor failure makes the image flicker, the Mintakans see inside, and one of them comes to believe that Picard is a god.
A larger example is seen in Star Trek: Insurrection, where Starfleet and the Son'a observe the Ba'ku village from a cloaked building, with staff moving about in isolation suits.
Data, damaged while on that assignment, reveals the blind to the villagers by firing on it, the first step in exposing the plan to remove the Ba'ku from their world.